Heat exchanger



FIG. 1 is a front elevational view of a heat exchanger, showing my new design;

FIG. 2 is a rear elevational view thereof;

FIG. 3 is a left side elevational view thereof;

FIG. 4 is a right side elevational view thereof;

FIG. 5 is a top plan view thereof;

FIG. 6 is a bottom plan view thereof;

FIG. 7 is a sectional view taken along line 7--7 of FIG. 1. 

The ornamental design for a heat exchanger, as shown and described. 